This invention relates to a method for the preparation of hydrophilic spongy gel-like materials. The invention relates more particularly to the production of spongy hydrophilic acrylonitrile-containing polymers and to articles produced therefrom.
Hydrophilic spongy gels as heretofore made, have been prepared by copolymerization of hydroxyethyl methacrylates with a minor amount of a glycol dimethacrylate in the presence of more than 40 % by weight of water. An excess of water separates in the course of the polymerization forming tiny droplets, which droplets communicate with each other if the polymerization is carried out in the presence of 70 % or more of water such that a true sponge-like substance having open pores is formed. The pores formed in this way, however, are too small for many purposes, for example where the spongy gel is to be used as an implantation material in medicine. In that case, a newly growing tissue needs enough room in the channels to be nutrified properly, or otherwise it withers away and calcifies. It is for the foregoing reason that there was made an attempt to enlarge the pores by the known method used in the production of viscose sponge materials, i.e., admixing into the polymerization mixture properly sized crystals which are soluble in water but insoluble in the polymerizing mixture, so that they can be washed from the product after the polymerization has been finished.
The above-mentioned methods and products thus formed, notwithstanding, have many failings which cannot be easily eliminated. First, the hydrophilic glycol methacrylate sponge has a low mechanical strength such being, especially sponges having large pores and/or considerable porosity. Second, the swelling capacity of the glycolmethacrylate polymer is limited (about 40 % by weight of water). Third, articles produced from the spongy hydrogel have to be shaped by polymerization casting, i.e., the threedimensional network of the polymer is formed by polymerizing the water-diluted monomer mixture in a proper mold. Fourth, the polymerization is very sensitive to the presence of traces of oxygen and other impurities acting as inhibitors of free-radical polymerization. Fifth, residues of the initiators, etc. (some being soluble in water but only slightly) have to be washed out thoroughly after the polymerization has been finished, and this accordingly requires a long and multi-step washing with water or costly washings with ethyl alcohol. It is known that there are stronger hydrogels than the polyglycolmethacrylate ones referred to above. There are, for example, copolymers of acrylonitrile obtained either by a partial hydrolysis of polyacrylonitrile or by copolymerizing acrylonitrile with a hydrophilic co-monomer(s). A spongy structure cannot be obtained in this case by phase-separation during the polymerization inasmuch as, if the acrylonitrile is polymerized or copolymerized in the presence of a precipitation agent, a so-called macroporous polymer having much smaller pores than those of the spongy one is formed in only the best case. If, on the other hand, a considerable amount of the precipitation agent is present during the polymerization, there is formed a heterogeneous structure consisting essentially of single-non-cohering polymer or copolymer particles. The polymer thus formed is easily crumbled as it does not possess the desirable strength and flexibility. Namely, the solid phase separates out during the polymerization instead of the liquid phase separation observed in the abovementioned case of glycol methacrylate polymerization.
It is an object of the instant invention to provide for a method of preparing spongy hydrophilic acrylonitrile-containing polymers.
It is another object of the invention to avoid one or more drawbacks of the prior art.
Other objects and advantages will become more apparent from the detailed description and claims which follow hereinafter.